A typical countercurrent pulp slurry washing system is depicted in FIG. 1 of Seymour U.S. Pat. No. 4,207,141 and is described at column 4, line 65 to column 6, line 2 of that patent and this depiction and description is incorporated herein by reference.
In this kind of system, the most commonly used method for controlling the amount of fresh wash water introduced via wash sprayer 1 of said FIG. 1 (i.e., into the last stage of the countercurrent operation) comprises measuring the percent solids in the wash liquor in the first stage filtrate tank (denoted 8" in said FIG. 1) normally on an hourly basis and controlling the amount of water introduced via said wash sprayer to obtain a target value for the percent solids in said first stage filtrate tank. Conventionally, the operator of the system also uses his knowledge of rate of pulp flow entering the system to adjust the flow rate to said spray washer 1 when changes in pulp entry rate occur. In stages different from the last stage, i.e., in the stages containing wash sprayers 1' and 1" in said FIG. 1, the flow rates through said sprayers are adjusted so as to maintain a relatively constant level in the filtrate tanks feeding said sprayers. This conventional method has the disadvantage that the system can contain excessive water in the takeoff stream to the evaporator and excessive water in the output stream which is required to be removed to a sewer, without detection. This is because an overwash part of the time cannot make up for an insufficient wash during another part of the time.
In another method for controlling the amount of fresh wash water introduction in said pulp washing process, conductivities in recirculating streams are measured and the fresh water introduction into the last stage (i.e., the shower flow rate via said sprayer 1) is adjusted to maintain a target value for each conductivity. This method is described in Rosenberger U.S. Pat. No. 4,096,028 and Sexton U.S. Pat. No. 4,046,621. A deficiency in this method is that accuracy depends on precise measurement of sodium ion content and conductivities are affected not only by sodium ion content but other ions present. Another defect in this method is that a change in dilution factor requires a substantial time period to effect a change in conductivity.
In another method for controlling the amount of fresh water introduction in said pulp washing process which is described in Canadian Patent No. 1,203,407, load on the washer drive mechanism is measured in an attempt to obtain a value proportional to the demand for wash water. This method has the deficiency that load on the drive mechanism depends on factors in addition to the amount of wash water required for a given dilution factor.
Still another method for controlling the amount of fresh wash water introduction into said pulp washing process is described in Seymour U.S. Pat. No. 4,207,141. In this method dielectric properties are measured in the pulp mat leaving the washing process to determine the total mass or its water content and wash water introduction is controlled in response to said determination. One problem in using this method is that available dielectric property measuring apparatus only measure such properties over 2 to 5 inches and the pulp mat leaving the process is normally 20-32 feet wide. Thus practicalities require measuring on preselected "sample" areas using stationary apparatus or on changing "sample" areas using apparatus which is reciprocated across the width of the mat. Such sampling does not give a total picture. Furthermore, a difficulty in using capacitance measuring means or microwave cavity perturbation means to conduct the capacitance measurements is in locating a capacitance electrode plate or microwave detection means below the mat. While use of a backscattered nuclear radiation device overcomes this difficulty and produces excellent results, the practical requirement for sampling referred to above and the fact that such apparatus does not detect radiation which is not backscattered means that efficiency of control could be improved.